1. Field of the Invention
The present invention relates to a liquid crystal modulation element capable of modulating light passing through a liquid crystal layer pixel by pixel and to a projection-type liquid crystal display apparatus for displaying an image through modulating light emitted from an optical projection system by a liquid crystal modulation element and enlarging and projecting the light onto a screen. The invention particularly relates to a liquid crystal modulation element that achieves color display with the single element and a projection-type liquid crystal display apparatus comprising such a single liquid crystal modulation element.
2. Description of the Related Art
Projection-type liquid crystal apparatuses such as liquid crystal projectors and liquid crystal projection television sets have been developed for enlarging and projecting an image on a liquid crystal panel onto a screen by means of an optical projection system through the use of the liquid crystal panel as an optical switching element or an optical modulation element. Such liquid crystal display apparatuses include a triple-panel apparatus comprising monochrome liquid crystal panels each provided in optical paths of blue (B), red (R) and green (G), respectively, and a single-panel apparatus comprising a liquid crystal panel having three color filters (CF) for selecting colors of B, R and G for every pixel. Since the triple-panel apparatus is large in size and less attractive in terms of costs, the single-panel apparatus with a simple configuration is often adopted where reductions in size, weight and cost are highly required.
The single-panel liquid crystal panel with color filters as described above has an advantage in that an image with excellent color purity is obtained. However, the color filters absorb much light and the light resistance is low. For example, the temperature of the filter for selecting a G color ray provided in correspondence with a pixel for G rises with the filter absorbing R and B color rays. The filter characteristics are thereby significantly reduced. Cooling to a sufficient degree is therefore required. The same applies to the R and B filters provided in correspondence with pixels for R and B. Furthermore, each color filter performs color separation through intercepting other color rays transmitted therethrough by absorption. Consequently, the efficiency of utilizing light from the light source is low and the light transmissivity of the liquid crystal panel as a whole is low. It is therefore difficult to efficiently perform cooling of the apparatus and to achieve high luminance with such a single-panel apparatus with color filters.
In order to overcome these problems, single-panel color liquid crystal display apparatuses are disclosed in, for example, Japanese Patent Application Laid-open No. 4-60538 (1992) that corresponds to U.S. Pat. No. 5,161,042 and `Asia Display '95 (p. 887)` wherein one condenser microlens is opposed to every three pixels. Three color rays of B, R and G are entered to each microlens from mutually different directions and condensed. The light sent out from the microlens is entered to each of the three pixels corresponding to three colors of B, R and G, respectively. Since such a projection-type liquid crystal display apparatus comprises a single liquid crystal panel with a microlens array instead of a color filter, the apparatus of this type will be called a projection-type liquid crystal display apparatus of the color-filterless single-panel microlens system. Since the apparatus does not comprise any color filter that would absorb a substantial portion of incident light, the apparatus exhibits an excellent light resistance and the liquid crystal panel as a whole achieves a high efficiency of utilizing light. Furthermore, the microlens allows light incident on regions between pixels (black matrix regions) to be effectively utilized as well. The substantial aperture ratio (the ratio of effective pixel area to the whole pixel area) is thereby increased and the efficiency of utilizing light is further enhanced. High illuminance of an image displayed is achieved, accordingly.
FIG. 1 is an enlarged cross section of a liquid crystal panel of a projection-type liquid crystal display apparatus of the color-filterless single-panel microlens system as disclosed in the above-mentioned publications and so on. The liquid crystal panel comprises: a pixel substrate 181 where a number of pixel electrodes are formed; a counter substrate 182 where counter electrodes and microlenses are formed; and a liquid crystal layer 183 placed between the pixel electrode 181 and the counter substrate 182.
The pixel substrate 181 includes: a glass substrate 1811; pixel electrodes 1812B, 1812R, 1812G and so on for B, R and G color rays, regularly (periodically) arranged on one side (on which light is incident) of the glass substrate 1811; and a black matrix section 1813 including thin film transistors (TFTs) (not shown) functioning as switching devices for applying voltage based on image signals to the pixel electrodes. The black matrix section 1813 is shielded from light with a metal film of aluminum and so on (not shown) so as to prevent the TFTs from wrongly operating.
The counter substrate 182 includes: a glass substrate 1821; a microlens array made up of condenser microlenses 1822 formed on one side (from which light goes out) of the glass substrate 1821; a cover glass 1823 placed in intimate contact with the microlenses 1822; and a counter electrode 1824 formed on the cover glass 1823. One of the microlenses 1822 is provided for every three pixel electrodes 1812B, 1812R and 1812G of the pixel electrode 181. The counter electrode 1824 is a transparent electrode formed all over the surface of the cover glass 1823 or in a required region of the surface of the cover glass 1823 (that is, at least in a region opposite the pixel electrodes 1812B, 1812R and 1812G of the pixel substrate 181). The counter electrode 1824 is maintained at a fixed potential.
The general operation of the liquid crystal panel shown in FIG. 1 will now be briefly described. The microlens 1822 condenses the ray bundles of B, R and G separated by dichroic mirrors not shown and entering from three different directions and have the ray bundles enter the pixel electrodes 1812B, 1812R and 1812G through the liquid crystal layer 183. The liquid crystal molecular orientation changes in the region of the liquid crystal layer corresponding to each pixel, depending on color image signals applied to the pixel electrodes of the pixel substrate 181. The three color rays of B, R and G incident on each region of the liquid crystal layer are selectively spatially modulated. The color rays thus modulated in the liquid crystal panel from an image by means of a projection lens not shown on a screen (not shown) and colors are synthesized. A color image is thus displayed on the screen.
Data projectors and rear projector televisions embodied through liquid crystal projection have been practically utilized. As the multimedia technology moves forward, the demand for apparatuses is expected that display a combination of a computer image and an audiovisual (AV) image with high definition as a high definition television. For such apparatuses, an optical system including a liquid crystal element that achieves higher definition, image quality and luminance is required, compared to conventional apparatuses. Consequently, it is required to reduce pixel areas.
With a reduction in pixel area, the diameter of a ray bundle condensed by a microlens is required to be reduced, accordingly. It is ideal that color ray bundles entering the microlens from the optical projection system in a preceding stage are completely telecentric ray bundles. However, actual incident color ray bundles include rays slightly shifted from the principal ray. As a result, color rays to enter one pixel only may be incident onto a black matrix section bordering on a neighboring pixel. The efficiency of condensing light is thereby decreased. Consequently, the luminance of a displayed image is decreased and the effect of the microlens is reduced. Furthermore, if the angle of shift from the principal ray mentioned above (the angle will be called incident divergence angle in the following description) is large, a color ray to enter a pixel for a specific color (a pixel for G, for example) may leak onto a neighboring pixel for another color (a pixel for R, for example). A phenomenon called color mixture thus occurs and color purity is reduced. Therefore, the luminance of a displayed image and color mixture are sensitively affected by the degree of incident divergence angle.
FIG. 2 shows color purity degradation due to color mixture, using the x-y chromaticity diagram of the XYZ color system of Commission Internationale de l'Eclairage (CIE). In the chart, coordinate points each indicated with a circle represent color rays of B, R and G without color mixture. Coordinate points each indicated with a square represent color rays into which another color ray is mixed by 1 percent. Coordinate points each indicated with a delta represent color rays into which another color ray is mixed by 2 percent. As shown, color mixture of a miniscule 1 to 2 percent results in a reduction in purity of color rays. In particular, the purity of R ray is significantly reduced.
As thus described, in the projection-type liquid crystal display apparatus of the color-filterless single-panel microlens system, if the incident divergence angle of a ray illuminating the liquid crystal panel is large, color mixture results and color purity of a displayed image is decreased and the image quality may be significantly reduced. It is therefore required to reduce the incident divergence angle of light incident on the liquid crystal panel to a sufficiently small degree. On the other hand, in order to reduce the incident divergence angle, it is required to sufficiently reduce the aperture provided in the optical projection system from the light source to the liquid crystal panel. As a result, the quantity of light reaching the liquid crystal panel is inevitably reduced and it is difficult to obtain a sufficient luminance of an image. That is, in such a projection-type liquid crystal display apparatus, there is a trade-off between improvements in color purity and luminance and it is difficult to achieve both at the same time.